The invention relates to the hot-top continuous casting of metals, especially steel. It relates more particularly to the hot-top casting of semifinished metals of elongate shape, such as slabs, thin slabs, etc., which are consequently cast in molds called molds of xe2x80x9cwide cross sectionxe2x80x9d.
It will be recalled that a mold of wide cross section is above all a bottomless metal tube which is disposed vertically and in which the cast steel solidifies on contact with the inner wall cooled vigorously by an intense circulation of water. This tubular element, conventionally made of copper, or more generally made of a copper alloy, has an ordinary height of the order of one meter. However, it differs from other types of continuous casting mold mainly by the fact that it is not monolithic but consists of four plates joined together at right angles: a pair of large plates, facing each other and intended to form the large faces of the cast slab, which is often much more than one meter in width, and a pair of small side plates mounted in line with the ends of the large plates in order to provide a seal with respect to the cast molten metal. Usually, for the sake of simplifying the vocabulary, these plates are called large or small xe2x80x9cwallsxe2x80x9d or, by analogy with the cast slab, large or small xe2x80x9cfacesxe2x80x9d.
Moreover, in its current state of development, the so-called xe2x80x9chot-topxe2x80x9d continuous casting of metals may be regarded, technically speaking, as an evolution of the conventional continuous casting process, which evolution shifts along the casting height at the level where the solidification of the metal in the mold occurs from the other level, located above it, where the free surface (or xe2x80x9cmeniscusxe2x80x9d) of the liquid metal in contact with the wall of the mold lies.
As is known, the first solidification takes place by a very sensitive physical mechanism and, at the same time, this represents an essential factor in the quality of the product obtained. By virtue of the heightwise separation of these two levels, which in conventional continuous casting are coincident or almost coincident, this solidification takes place in a hydrodynamically calm location, remote from the always disturbed zone of the meniscus region. Specifically, this separation of the two levels is obtained by surmounting the cooled copper body of the mold with an attached feed head, which is not necessarily cooled. The feed head is typically made of a refractory material having high thermally insulating properties and is internally well aligned with the mold to keep, throughout the duration of the casting operation, the meniscus of cast steel poured from a tundish placed just above it.
Hot-top continuous casting of this type, although known for a long time according to these principles, as described for example in EP-A-0620062, has at the present time, to the Applicant""s knowledge, still not achieved industrial realization. The studies carried out more recently by the Applicant on the subject (see for example FR-A-2747061 and FR-A-2747062) have, however, shown the great advantage of providing, in the bottom part of the insulating refractory feed head, an insert made of a dense refractory material which is mechanically much stronger than the usual insulating refractories. This inserted piece must in fact simultaneously be a good heat insulator, in order to keep, like the feed head, the molten steel that it contains in the liquid state, and have good mechanical strength properties in order to retain for as long as possible the geometry of the upper edge of the copper wall on which it rests, specifically at the point where the solidification of the cast metal is initiated. It is known that a material such as SiAlON (R) satisfies such requirements quite well. However, this type of material is expensive, particularly when it has to be shaped into a ring matching the inner perimeter of the mold. Further, the cost may even become prohibitive for long inserts, as is necessarily the case for molds of wide cross section.
Moreover, it is exceedingly important for the success of the casting operation to maintain strict alignment of the SiAlON insert with the large faces of the mold which are placed beneath it within very narrow tolerance margins, of the order of {fraction (1/10)}th of a mm. Such a requirement is all the harder to meet since the inevitable hot differential expansion phenomena of the elements present in contact with the molten metal are a major cause of misalignment. In addition, it should be noted that such phenomena are of greater consequence the larger the size of the mold, something which is particularly the case, here again, when casting steel slabs (the width conventionally being able to reach or even exceed 2 m).
Now, this alignment requirement specific to hot-top continuous casting is not well met by the current technology of molds of wide cross section. The current mold technology relies schematically on the principle of the stiffening backplate. Each large copper plate bears on a stiffening plate which is matched to it, being fastened thereto by assembly with the aid of transverse ties anchored in the copper plate and distributed over the height and over the width of the plate with inter-tie spacings of some twenty cm approximately. The robustness of such an assembly cannot be disputed. However, during casting, in other words when xe2x80x9chotxe2x80x9d, it results in a wavy deformation of the copper plate between each tie. Admittedly, this deformation, of a few tenths of a mm at most, is of no real consequence in conventional continuous casting, but it is totally unacceptable in hot-top continuous casting because of the misalignment that it causes between the copper plate and the refractory feed head in their mating plane, at precisely the point where the first shell of solidified cast metal is formed.
The object of the present invention is to provide a simple, reliable and economic solution to the aforementioned difficulties encountered with the hot-top continuous casting of products of wide cross section.
For this purpose, the subject of the invention is a mold of wide cross section for the hot-top vertical continuous casting of metals, particularly steel, comprising a tubular body formed by an assembly of copper, or copper alloy, plates cooled by the circulation of a coolant, this cooled metal tubular body being surmounted by a feed head made of a thermally insulating refractory material, internally aligned with it and having, among the plates of which it is composed, large plates each fastened to stiffening backplates with the aid of distributed transverse ties, wherein the upper part of each of said large plates has a shoulder set back with respect to the plane of the large plate in order to be able to engage with the jaw of a clamp of elongate shape, which clamp thus fastens the upper part of the large plate against the associated backplate, by going over the top of the latter and providing a continuous grip distributed over the width of the large plate, the base of the refractory feed head resting against that part of the shoulder left clear by the jaw of the clamp, the other jaw of the clamp being provided with means for adjusting the clamping by bearing against the associated backplate.
As will have doubtless been understood, the ties of the upper part of the mold are replaced by a clamp whose jaw, and therefore the bearing surface, may also be extended as required along the upper edge of the large plates, or even continuously, so as to completely counteract the vague tendency of the upper edge of the large copper plates to deform, this point in the mold being very sensitive in the success of hot-top casting, as explained above. This clamp goes over the top of each large plate and its associated backplate. A step, set back from the upper edge of the large plate, is thus designed to allow the jaw of the clamp to be positioned in the shoulder thus provided at the top of the mold in order to bear on the heel of the copper plate and consequently clamp it rigidly against the base plate on which the other jaw of the clamp acts. This modification of the shape of the upper part of the large plates according to the invention is reminiscent of the shape at the xe2x80x9chead of the femurxe2x80x9d except that, however, the offset head is intended, not for pivoting in an accommodating cupola, but to serve for a locking clamp to be engaged against the backplate.
Since the step is intentionally larger in size than the jaw of the clamp which it houses, the space thus left free on the shoulder in front of the jaw constitutes an alignment housing for easily positioning a refractory feed head.
It is consequently possible, much more easily than hitherto, to make the insert, provided at the base of the feed head, from a hard xe2x80x9cSiAlONxe2x80x9d refractory. Thus, this construction, by the assembly of juxtaposed sections (FR-A-2764533) whose alignment with respect to one another is the guarantee of the indispensable rectitude of the entire assembly, is carried out, according to a secondary subject of the invention, from sections of refractory material, each one of which is gripped hot in a metal shell which provides them with the desired mechanical strength and which are mounted in a strip on a rigid guide rod until the desired length is obtained (equal at most to the width of the large plate) and at the ends of which rod resilient clamping means are provided so as to keep the sections clamped together.